Fluid catalytic cracking, (hereinafter referred to as FCC), is a well-known petroleum process and need not be described herein in detail. Briefly, the FCC process is operated in a plant comprising a cracking unit that has a cracking zone operating in the absence of added hydrogen and a regenerator that has a regeneration zone designed to combust organic matter. Such plants contain a circulating inventory of solid acidic cracking catalyst which passes through the cracking zone where fresh feed such as a petroleum gas oil is converted to more useful products such as gasoline and fuel oil with concomittant formation of coked catalyst. The coked catalyst is usually stripped with steam to remove volatiles and then passes to the regenerator where the coke is removed by combustion with formation of a flue gas and regenerated catalyst which is returned in continuous fashion to the cracking zone. All references made herein to "coked FCC catalyst" are intended to refer to the catalyst after conventional steam-stripping. Many patents have issued concerned with one or more aspects of the FCC process, some of these dealing with process variations, others with hardware, and others with control of the process, including control of CO emmissions. Schwartz U.S. Pat. No. 4,159,239, is exemplary of such patents, and is incorporated herein by reference as if fully set forth.
Regenerators in FCC plants produce copious quantities of flue gas which are directly or indirectly discharged to the atmosphere. Because of the large scale that is characteristic of such plants, the environmental impact of the flue gas discharged is of considerable importance. In particular, the discharge of carbon monoxide in excessive quantities, as well as nitrogen oxides and oxides of sulfur have come under scrutiny and one or another of these have become the subject of restrictive legislation.
For convenience, the noxious oxides of nitrogen and sulfur will be referred to herein as NO.sub.x and SO.sub.x, respectively, and the carbon monoxide as CO.
As much as half of the nitrogen and sulfur in the FCC feed is incorporated into the coke deposited on the catalyst in the FCC reactor. Upon regeneration of the catalyst, the incorporated nitrogen and the sulfur are burned off along with the coke to form NO.sub.x and SO.sub.x which are emitted in the flue gas from the regenerator. In addition, CO can be emitted in these gases as a result of incomplete combustion within the regenerator.
With the changing quality of crude oils available today, FCC feeds often have increased levels of nitrogen and sulfur (as high as 5000 ppm and 2.5%, respectively). These levels translate into regenerator NO.sub.x and SO.sub.x emissions which can be significantly higher than current or proposed limits set by governing municipalities. It is therefore likely that many FCC units will require some means to control such emissions.
In a typical single-stage regenerator, the operating parameter which most influences NO.sub.x and SO.sub.x emissions is the excess O.sub.2 in the flue gas. Most problematic is that increasing the excess O.sub.2, which is desirable to achieve a high degree of coke burnoff and CO conversion, greatly increases the emissions of both NO.sub.x and SO.sub.x. However, it has been found previously that SO.sub.x emissions can be significantly reduced under excess O.sub.2 conditions by incorporation of a "SO.sub.x -transfer agent", such as alumina, or rare earths on alumina, with the FCC catalyst. The use of alumina for such purpose is described in Flanders et al., U.S. Pat. No. 4,115,251 the entire contents of which are incorporated herein by reference as if fully set forth. The use of rare earths on alumina is described in Bertolacini et al., U.S. Pat. No. 4,369,108 the entire contents of which are incorporated herein by reference. For these catalysts to perform well it is necessary to maintain a high level of excess O.sub.2, typically more than 2 vol %, in the flue gas. For this reason, it has not been possible to achieve a simultaneous reduction in NO.sub.x when a SO.sub.x transfer agent is employed; in fact, NO.sub.x emissions usually increase.
Most FCC units now use zeolite-containing catalysts having high activity and selectivity. Zeolite-type catalysts have a particularly high activity and selectivity when the concentration of coke on the catalyst after regeneration is relatively low, such as below about 0.1 wt %, so that it is generally desirable to burn off as much coke as possible in regeneration. Thus, any solution to the flue gas emissions problem which involves the design or operation of the regenerator preferably should favor production of clean-burned regenerated catalyst.
The problem of carbon monoxide emissions has been addressed and a very widely accepted solution has been to include in the catalyst inventory a trace amount of a platinum group metal, as described in the above-cited Schwartz, U.S. Pat. No. 4,159,239 which promotes the combustion of carbon monoxide in a controllable manner in the dense-phase fluidized catalyst bed in the catalyst regenerator, the dense bed providing an adequate heat sink for recovery of process heat. The wide industrial acceptance of platinum promoted cracking catalysts lies not only with its effectiveness in reducing carbon monoxide emissions, but also because this mode of control fosters clean burning of the catalyst. This clean burning is effected in part because the recovered heat increases the catalyst bed temperature in a controlled fashion, and also because a high level of residual oxygen in the flue gas is permitted without danger of encountering afterburning. However, a problem encountered in some cracking operations using metal promoted, complete carbon monoxide combustion-type regeneration has been the generation of undesirable NO.sub.x in the flue gas. Thus, we see another incompatibility of the type in which the abatement of one pollutant, CO, leads to the aggravation of another pollutant, NO.sub.x.
It is an object of the present invention to provide a novel concept FCC regenerator particularly well suited for the continuous regeneration of coked cracking catalyst formed in a FCC unit. It is a further object of this invention to provide a method for regenerating coked FCC catalyst and producing a regenerator flue gas having ecologically acceptable levels of NO.sub.x. It is a still further object of this invention to provide a method for regenerating coked FCC catalyst with reduced emissions of any two or more of SO.sub.x, NO.sub.x and CO.
These and other objects of this invention will become evident on reading this entire specification including the appended claims.